In these days, optical input and output apparatus, optical transmission apparatus, optical switching apparatus, optical signal processing apparatus, etc. having a high signal capacity and a property of high speed are required for practical uses, since the information society has become highly advanced. In this point, a computer consisting of electronic circuits has a limitation. Therefore, an optical computer using the high speed of light and the parallel property thereof is expected to be developed. On the other hand, a neural network using optical interconnection is intensively studied to provide a practical structure. In this neural network, unit elements each defined as a "neuron" are connected by interconnections, so that an output signal is obtained from input signals by changing coupling degrees among neurons. That is, all of input signals are added to provide a sum which is compared with a threshold value to provide an output signal.
At the present stage, one type of a neural network is structured by integrated electronic devices. In this neural network, an interconnecting portion of the electronic devices may be replaced by optical devices. Another type of neural network is structured totally by optical functional devices. In any structure, an extremely large number of interconnections must be done to provide a large scaled neural network. In this point, light provides effective means for interconnections among neurons of a neural network, because the spatial transmission of information can be carried out by use of light, and a high density of interconnections can be realized due to no mutual interference of light signals.
In an optical neural network, spatial light modulating devices must be provided to change a coupling degree for each interconnection. For this purpose, liquid crystal displays are only used in the optical neural network. However, the liquid crystal displays have a disadvantage in that an operating speed is ordinarily as slow as several m sec., so that a high speed operation can not be realized. Otherwise, such devices as using non-linear materials or semiconductors are utilized as optical processing devices having high operating speeds in the optical neural network. However, these devices have a disadvantage in that the high density formation of a device arrangement is difficult to be realized, and because a technology for arranging the devices in a predetermined pattern is not sufficiently developed, and electric power consumption is large.
In this situation, attention has been paid to an optical functional device with low electric power consumption which is based on an optical thyrister. This device has been described in a report entitled "A new double heterostructure optoelectronic switching device using molecular beam epitaxy" on pages 596 to 600 of "Journal of Applied Physics, Vol. 59, No. 2, 1986", and in a report entitled "Double heterostructure optoelectronic switch as a dynamic memory with low-power consumption" on pages 679 to 681 of "Applied Physics Letter, Vol. 52, No. 9, 29 February, 1988".
Arbitrary connections using optical interconnections having a large number of parallel connections which have been carried out by input signals are realized by using functions of light emission, light response, and information storage of the above described optical functional device.
Here, an optical interconnection apparatus using a matrix of optical functional devices will be explained, as described in a paper "3a-ZA-2" on page 795 of "Extended abstracts, The 36th Spring Meeting, 1989, The Japan Society of Applied Physics and Related Societies, No. 3" held on April 1 to 4 of 1989. This optical interconnection apparatus comprises a matrix of optical functional devices and a light receiving structure. The matrix includes NXN optical functional devices arranged in a matrix pattern, and the light receiving structure includes N light receiving devices arranged in a one-dimensional pattern to face the NXN optical functional devices. In this optical interconnection apparatus, arbitrary optical interconnection are realized by driving predetermined optical functional devices which are selected from the NXN optical functional devices by applying predetermined signals to signal lines selected from N X-axis lines and N Y-axis lines. In this operation, each optical functional device has an ON state and OFF state corresponding to binary code states of "0" and "1", so that the optical functional devices operate as spatial light modulators. Thus, spatial light modulated signals are supplied to the light receiving devices. This optical interconnection apparatus has an advantage in that it operates with a speed much higher than that of an optical interconnection apparatus using liquid crystal displays.
However, this optical interconnection apparatus has a disadvantage in that coupling degrees are only set in the connection of information by the binary values of "0" and "1". Considering that this optical interconnection apparatus is applied to an optical neural network, an optical arithmetic and calculating apparatus, etc., it is desired that coupling degrees change continuously between "0" and "1".